Not wishing to tread on Matt's toes, but if you "google" Barbour clothing and go to the glove section you will see mittens for sale. Almost standard issue for farmers, outdoors-men, shooting types and fishermen in the UK and rightly so.

I don't remember where I got them, but I have a pair of wool mittens with a fold back cover held back by velcro so that when the tips of the fingers are needed they can be easily uncovered. First class kit.